1. MOLECULAR GEOMETRY Determining the Structure of Molecules

2. Molecular Structures Molecular Formula H 2 O Electron Dot Structure H:Be:H Structural FormulaH – Be – H Ball and Stick Molecular Model

3. MOLECULAR GEOMETRY Structural formulas, such as NH 3, provide information about bonding only. It does not provide direct information about the shape of the bond or the shape of the molecule. The repulsion between charge clouds in the outer levels of atoms determines the arrangement of the orbitals. The orbital arrangement determines the shape of the molecules.

4. VSEPR Valence Shell Electron Pair Repulsion theory is based on the number of regions of high electron density around a central atom. can be used to predict structures of molecules or ions by minimizing the electrostatic repulsion between the regions of high electron density. can also be used to predict structures of molecules or ions that contain multiple bonds or unpaired electrons. does fail in some cases.

5. VSEPR In small molecules, electron pairs will spread as far apart as possible to minimize repulsive forces. Two electron pairs = 180  apart Three electron pairs = 120  apart Four electron pairs = 109.5  apart

6. SHAPES WE WILL LEARN Linear Trigonal Planar Tetrahedral Pyramidal Bent

7. LINEAR Atoms are connected in a straight line. 180  bond angles One or two bonded pairs of electrons Examples:HClCO 2

8. TRIGONAL PLANAR Atoms are connected in a flat equilateral triangle Three bonded pairs of electrons 120  bond angle Example:BCl 3

9. TETRAHEDRAL Atoms are connected in a shape with four surfaces. Four bonded pairs of electrons 109.5  bond angles. Example: CH 4

10. PYRAMIDAL Atoms are in the shape of a pyramid. Similar to tetrahedral but only has three bonded pairs of electrons, not four; has one unshared/lone pair of electrons. Less than 109.5  bond angle due to unshared pair of electrons. Example: NH 3

11. BENT Atoms are close to the shape of a tetrahedral, but the two unshared pairs of atoms exert a greater repulsive force than the two sets in the bonds. Two shared pairs of electrons and two unshared pairs of electrons 105  bond angle Example: H 2 O

12. Other Shapes

13. VSEPR TABLE Look on the back of your Molecular Geometry worksheet

14. HOW TO DETERMINE THE GEOMETRY OF A MOLECULE USE ELECTRON DOT DIAGRAMS 1. Look at the chemical formula. Figure out the location of the atoms Hydrogen is always on the outside The least electronegative atom is the central atom (most “electropositive”). 2. Draw the Electron Dot Diagram for each atom. 3. Count up the total amount of valence electrons for all the atoms involved.

15. HOW TO DETERMINE THE GEOMETRY OF A MOLECULE 4. Determine the number of bonding pairs of electrons by dividing the total # of electrons by two. 5. Arrange a skeletal diagram of the molecule by placing the other atoms around the central atom. Place a bonding pair of electrons (2) between the central atom and each of the terminal atoms.

16. HOW TO DETERMINE THE GEOMETRY OF A MOLECULE 6. Subtract the number of bonded pairs of electrons for the central atom from the total number of electrons. If there are any electrons left over, these electrons are lone pairs to be placed around the central atom.

17. HOW TO DETERMINE THE GEOMETRY OF A MOLECULE 7. If the terminal (outside) atoms do not have a full octet, place lone pairs of electrons around them. The rest go around the central atom. 8. If the central atom does not have four pairs of electrons around it (and it had a least four valence electrons to begin with), try converting some of the lone pairs to double or triple bonds. (Carbon, nitrogen, oxygen, and sulfur like to form multiple bonds)

18. EXCEPTIONS There are exceptions to the OCTET rule: Atoms with less than an Octet: Hydrogen – only 2 valence electrons Group 2A – only 4 valence electrons Group 3A – only 6 valence electrons Atoms with more than an Octet: Sulfur and phosphorus – 10+ valence electrons Krypton, xenon, iodine, and others with “d” orbitals will accept more than 8.

19. PRACTICE Fill in the following chart and predict the molecular shape for the following substances: MoleculeE- Dot diagram# of shared/# of lone/Electron DotShape ofBond Type Formulaall elementsbonded e-unshared e-StructureMolecule(e-negativity)

20. PRACTICE H 2 O SiCl 4 NH 3 Cl 2 N 2 GaF 3

22. MOLECULAR POLARITY This is a result of bond dipoles (areas of unequal polarity) that do not cancel each other out. This is the polarity of the MOLECULE not the BOND.

23. BOND POLARITY You can determine the polarity of BONDS by determining the electronegativity differences of the two atoms involved. C – Cnonpolar cov.e-neg diff = 0 Na – Fionice-neg diff – 3.05 C - Hnonpolar cov.e-neg diff = 0.35

24. MOLECULAR POLARITY But, take those same molecules and the polarity of the molecule will depend on the whole molecule, not just the bond. C – Cnonpolarequal sharing Na – Fpolarunequal sharing C - Hpolarunequal sharing For a molecule, you must consider the shape and the terminal atoms.

25. MOLECULAR POLARITY LINEAR If the terminal atoms are the same, there are equal forces, so it is NONPOLAR. If they are not the same, it is POLAR. BeF 2 – nonpolar HCl - polar

26. MOLECULAR POLARITY TETRAHEDRAL If the terminal atoms are the same, there are equal forces, so it is NONPOLAR. If they are not the same, it is POLAR. CCl 4 – nonpolar CHCl 3 - polar

27. MOLECULAR POLARITY TRIGONAL PLANAR If the terminal atoms are the same, there are equal forces, so it is NONPOLAR. If they are not the same, it is POLAR. BCl 3 – nonpolar BHCl 2 - polar

28. MOLECULAR POLARITY PYRAMIDAL Because of the unshared pair, there are unequal forces, so the molecule is POLAR. NH 3

29. MOLECULAR POLARITY BENT Because of the unshared pairs, there are unequal forces, so the molecule is POLAR. H 2 O

30. MOLECULAR POLARITY

31. What is the Molecular Polarity for these molecules? Molecular Polarity

32. What about these?

33. REMEMBER! To determine BOND POLARITY, calculate the electronegativity differences. To determine MOLECULAR POLARITY, look at the shape of the molecule and the terminal atoms.

34. ORBITAL HYBRIDIZATION This is the mixing of atomic orbitals in an atom to generate a new set of atomic orbitals. S and p orbitals merge and there no longer are distinct orbitals. They merge to form sp orbitals.

35. ORBITAL HYBRIDIZATION

38. Determine the Hybridization: sp 3 sp sp 2

39. Do this now – scrap paper Molecule Formula E- Dot Diagram for all atoms # shared/ bonded electrons # lone/ unshared electrons E- Dot Structure Shape of molecule Bond Type (electro- negativity) AlH 3 PH 3 CS 2